Personal Communication Service (PCS) is largely an enhancement of cellular radio telephone service which, in turn, is based on early two-way radio systems. In the simplest configuration, a receiver and transmitter share a common base station antenna for receiving and transmitting signals to and from mobile stations. Typically, the receiver and transmitter each use a separate frequency range for sending and receiving information. By grouping multiple pairs of frequency ranges, called frequency blocks, multiple mobile stations can share the same base station antenna infrastructure and provide advantages to the system operator.
One or more transmitters/receivers may be coupled to the antenna to accommodate different frequency blocks or different frequency ranges within a block. Current coupling implementations use components functioning as couplers, splitters, and duplexors to couple the antenna with the receiver and transmitter. Couplers typically combine two separate signals from two sources into one output source. In contrast, splitters accept one input signal and provide two nearly identical signals on separate outputs. Duplexors allow an input and an output signal on a common source to be separated at a common point into separate input and output channels.
FIG. 1 is a functional block diagram that illustrates a typical prior coupling system 100 for two transmitters and two receivers sharing a common antenna in a commercial mobile radio service (CMRS) system. Referring now to FIG. 1, an antenna 110 receives electromagnetic signals originating from mobile stations 125 and transmits such signals to the mobile stations. A hybrid coupler/duplexor 105 functions both as a coupler for combining signals from multiple transmitters 103 and as a duplexor to separate received signals 130 from transmit signals 128.
The received signals from antenna 110 are output to the coupler/duplexor 105, separated from the transmit signals 128, and sent to a splitter 115. The splitter 115 divides each received signal 130 output by the duplexor 105, thereby providing two nearly identical signals 135 to receivers 120. Because each output signal 135 contains all frequencies of the electromagnetic signals received by the antenna 110, each receiver 120 typically filters unwanted signals. The transmit signals 128 originate from transmitters 103 and are combined by the coupler/duplexor 105 for transmission by the antenna 110.
The duplexor function of the coupler/duplexor 105 separates the received signals from the transmit signals. Duplexors are traditionally assembled by connecting mechanically-tuned cavities together in a bandpass, band reject, or hybrid configuration. All duplexors must exhibit certain characteristics if optimum system performance is to be achieved. As shown in Table 1, these typical duplexor characteristics include:
TABLE 1(1)Operate in the specified transmit and receivefrequency bands.(2)Handle total transmit power.(3)Provide adequate rejection of transmitter noise in thereceive band.(4)Provide adequate isolation between the transmitterport and the receiver port to prevent receiverdesensitization.(5)Provide minimum insertion loss, which is dependentupon the duplexor design and the frequencyseparation between the transmit and receive bands.
The coupler function of the coupler/duplexor 105 is also usually constructed of mechanically-tuned cavities designed to provide low transmission losses. The splitter 115, on the other hand, is usually constructed with resistive components and has built-in amplifiers to compensate for any additional signal losses.
While the prior coupling system 100 is proven and well known, this coupling system carries the penalty of increased transmission losses as multiple transmitters are added. This addition of transmitters also requires the insertion of multiple couplers, which degrades the signal due to increase thermal noise. Because the components pass wideband signals, they offer no inherent advantages for adjacent signal rejection.
The configuration shown in FIG. 1 is the conventional method of coupling transmitters and receivers in certain CMRS systems, such as AMPS-compatible systems. Because only two licensed cellular carriers are allowed in a given AMPS market, each cellular service provider usually deploys its own antennas and infrastructure. In contrast, PCS regulators allow up to six licensed providers in each PCS market. Because of increased difficulty in obtaining approval for constructing towers in urban areas, PCS providers now often share the same towers for mounting antennas. As the power of the received signal is proportional to the distance between the transmitting and received antenna, co-location leads to one service provider's transmitted signals being received by another provider's antenna. Because of the multitude of providers operating in adjacent frequency blocks, as well as the close physical proximity of antennas, blocking adjacent signals is of greater concern to PCS providers than to cellular providers. Although conventional coupling technology can be used in PCS equipment to couple receivers and transmitters, prior coupling systems do not offer satisfactory signal interference rejection levels and signal to noise ratios for PCS systems. Moreover, the prior coupling systems suffer from increased transmission losses when multiple transmitter elements are added to the typical PCS system implantation.
In view of the foregoing, there is a need for an improved system to couple a transmitter and receiver of a CMRS system, such as a PCS system, to an antenna. The present invention provides a coupling system for CMRS systems that overcomes the disadvantages of the prior art while offering improved interference signal rejection, lower transmission losses, and improved signal-to-noise ratios.